Nucleic acids show great promise as new therapeutics to treat both acquired and inherited diseases. One of the greatest challenges with the successful application of nucleic acid drugs is the development of an efficacious delivery method.1 Delivery systems are needed to compact genetic material into nanostructures that can be taken up by cells, protect nucleic acids from enzymatic damage during cellular transport, and provide the possibility of targeting the delivery to specific cell types.2 Viral vectors are still the most effective and commonly used method of DNA transport even though many problems with this delivery method have been revealed.2,3
Polymer-mediated gene delivery has recently emerged as a viable alternative to viral-based transduction systems since polymers may not induce immune and inflammatory responses, have a lower cost of synthesis, and have a large nucleic acid loading capacity.1,2 Several studies have shown that polycations bind DNA electrostatically and form polyplexes (polymer+DNA complexes) that are endocytosed by many cell types and deliver DNA with varying degrees of delivery efficiency and toxicity.4,5 Although synthetic delivery systems show great promise, difficulties with polymer toxicity and low delivery efficiency have hampered clinical application of these vectors.1,2 For example, polyethylenimine (PEI), a polymer of ethylenediamine, exhibits efficient gene delivery but is also very cytotoxic.6 Conversely, chitosan, a polymer of glucosamine, is completely nontoxic yet reveals low delivery efficiency in many cell lines.7 Progress towards rationally-designed synthetic delivery systems has also been stalled by a lack of understanding of the fundamental polymer structure-biological property relationships that exist for synthetic delivery vehicles.4,5
Drug delivery is an important field for both clinical applications and research. Some biological systems possess unique delivery challenges.
In recent years gene therapy has received a greater amount of attention in academic and scientific circles. The potential for gene therapy for pharmaceutical, commercial, and clinical applications is tremendous. Gene transfection, the addition of a gene to a cell, is a critical component of gene therapy.
Presently there are several approaches to gene transfection. These include the use of viral based vectors (e.g., retroviruses, adenoviruses, and adeno-associated viruses) (Drumm, M. L. et al., Cell 62:1227-1233 (1990); Rosenfeld, M. A. et al., Cell 68:143-155 (1992); and Muzyczka, N., Curr. Top. Micro. Immuno. 158:97-129 (1992)), charge associating the DNA with an asialorosomucoid/poly L-lysine complex (Wilson, J. M. et al., (1992)), Charge associating the DNA with cationic liposomes (Brigham, K. L. et al., (1993)) and the use of cationic liposomes in association with a poly-L-lysine antibody complex (Trubetskoy, V. S. et al., Biochem. Biophys. Acta 1131:311-313 (1993)).
Viral vectors have exhibited the highest levels of transfection efficiency to date for nucleic acids. Viral vectors have been particularly effective in in vivo systems, where other transfection systems have fallen short. Viral vectors do have a tremendous downside, namely the potential to illicit a potentially life-threatening immune response. (Kingman, Bioworld Int., 1 (20): 1 (1996)). This happens because the viral carrier actually infects the cell as part of the method of transfection.
Although non-viral based transfection systems have not exhibited the efficiency of viral vectors, they are still receiving significant scientific attention because of their probable increased safety for in vivo systems. This has also led to increased attention for in vitro systems as well. Synthetic cationic molecules have been reported to “coat” the nucleic acid through interactions on the cationic sites of the transfection reagent and the anionic sites on the nucleic acid. The positively charged coating reportedly interacts with the negatively charged cell membrane to facilitate the passage of the nucleic acid into the cytoplasm via non-specific endocytosis. (Schofield, Brit. Microencapsulated. Bull., 51(1):56-71 (1995)).
Past attempts at nucleic acid transfection have also experienced difficulty with DNA precipitating out of solution. The problem is especially acute in in vivo applications where typically higher concentrations of DNA are present. These higher concentrations create solubility problems for the DNA/carrier systems. DNA precipitation can be avoided by increasing the concentration of mono- and polyvalent cations. In the past this had partly solved the DNA solubility problem, but it also increased the toxic effects upon the transfected cells.